In the case of transmitting a signal of a millimeter waveband, a waveguide is sometimes used. A signal transmitted by the waveguide passes a signal transmission line such as a microstrip line and is inputted into a semiconductor. Generally, a quarter wavelength antenna line and a back-short structure are adopted at a connecting portion between the waveguide and the signal transmission line.
When a semiconductor package including the back-short structure is mounted with a semiconductor device having a large amount of heat generation, such as a semiconductor chip for signal power amplification, the semiconductor package is required to have high heat radiation characteristics. However, according to a semiconductor package in a related art which is lightweight and low cost, sufficient heat radiation characteristics cannot be obtained because the package has pseudo-waveguide in a dielectric substrate and the semiconductor device is mounted on the dielectric substrate.
Further, to reduce weight of a final product, the semiconductor package is desired to be lightweight. However, according to the semiconductor package in the related art, the back-short structure is heavyweight because of being made of metal.